Location Enhancement to IEEE DCF

Transcription

1 Location Enhancement to IEEE DCF Tamer Nadeem 1, Lusheng Ji 2, Ashok Agrawala 1, and Jonathan Agre 2 1 Department of Computer Science University of Maryland College Park, Maryland {nadeem,agrawala}@cs.umd.edu 2 Fujitsu Labs of America 8400 Baltimore Ave., Suite 302 College Park, Maryland {lji,jagre}@fla.fujitsu.com CS-TR-4606 and UMIACS-TR June 2004 Abstract In this paper, we propose an enhancement to the existing IEEE DCF MAC function to improve channel spatial reuse efficiency, and thus improve data throughput. Our modification, named the Location Enhanced DCF (LED) for IEEE , incorporates location information in the RTS-CTS-Data-ACK handshakes of the IEEE DCF so that other stations sharing the channel are able to make better interference predictions and blocking assessments. Physical layer receiver capture effect is also used in LED to encourage more concurrent transmissions, after the result of the improved channel estimation. In this paper we analytically study the potential performance enhancement of the LED over the original IEEE DCF. The results are also verified using the ns-2 simulator, which show that up to 35% of DCF blocking decisions are unnecessary and our LED method can achieve up to 22% more throughput than the original DCF. 1 Introduction The IEEE [1] is the most popular standard for Wireless Local-Area Networks (WLANs). The IEEE Medium Access Control (MAC) specifies two different mechanisms: the mandatory contention-based Distributed Coordination Function (DCF) and the optional polling-based Point Coordination Function (PCF). At present, DCF is the dominant MAC mechanism implemented by IEEE compliant products. 1

2 Contention based protocols are the mainstream MAC protocols for distributed and self-organized wireless networks since in such networks the infrastructure is usually not present and there is no clear separation between the roles of access points and client stations. The support of contention based DCF also made IEEE equipment popular choices for various wireless ad hoc networks. Most of the contention based MAC protocols, including the IEEE DCF, are based on Carrier Sense Multiple Access (CSMA). In CSMA, a station may transmit if and only if the medium is sensed to be idle. The reason is to avoid this station causing interference to the current ongoing transmission sensed on the medium. In addition to basic CSMA, the DCF also incorporates acknowledgement signals and a back-off mechanism. An optional channel reservation mechanism is also include in the DCF. The IEEE DCF is known to be not efficient in shared channel use due to its over-cautious approach towards assessing the possibility of causing interference. In particular, a station simply blocks its own transmission when it senses the medium is busy, or it receives a channel reservation message sent by any other station. However in many cases this channel assessing station s own transmission may not introduce enough signal energy to disturb the on-going transmission at its receiver. Finer channel assessment schemes which do consider the above possibility are difficult to implement with information provided by the current IEEE communication protocol. If more information regarding an on-going transmission, such as locations of the transmitters and receivers and transmission power levels can be provided to the channel assessing stations, it is then possible for these stations to make better estimations to decide if indeed their transmissions will collide with the on-going transmission. In this way, more concurrent transmissions in wireless networks may be encouraged and the communication channel can be used more efficiently. In this paper, we propose a novel contention-based distributed MAC scheme which assesses the channel condition more accurately and exploits radio signal capture phenomena to increase the simultaneity of data transmissions to enhance wireless network performance. This scheme is designed as an enhancement to the DCF. In doing so, we will also develop a new MAC frame format in addition to the new MAC protocol to provide the additional information to help the stations in deciding whether to block their transmissions or not, when there are on going communications occurring in their vicinities. 2 Backgrounds and Related Works 2.1 IEEE DCF Mode Historically, the design of the IEEE DCF is influenced by several other protocols. MACAW protocol [4], extending the predecessor Multiple Access Collision Avoidance (MACA) protocol [14], is based on the use of the Request-to-send and Clear-to-Send (RTS/CTS) handshaking scheme. If a node has a packet to send, it first transmits a RTS packet to request the channel. If available, the receiver replies with a CTS packet. After the sender receives the CTS packet successfully, it proceeds to transmit the actual data packet. Nodes that hear the RTS packet will defer transmission 2

3 for a sufficiently long period of time to allow the transmitter to receive the CTS packet. Nodes hear the CTS packet will back off for a period of time that is sufficiently long to allow the receiver to receive the entire data packet. Sender nodes in such mechanism do not use the carrier sense mechanism to assess the channel availability. An extended protocol named Floor Acquisition Multiple Access (FAMA) is proposed in [8]. FAMA bears significant resemblance to IEEE , employing both local carrier sense, as well as the RTS/CTS collision avoidance exchange for data transmission. The basic MAC method of IEEE , the DCF, is a Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism with a random back-off time window after sensing a busy medium. ACK messages are also used in the DCF for acknowledging the reception of unicast data frames. The CSMA scheme of the DCF works as follows. Before a station transmits, it must sense the wireless channel to determine if any other stations are transmitting. The channel is assessed as busy if there are detected carrier signals or the energy level of the channel exceeds a threshold, or both, depending on each particular vendor s implementation. If the carrier is assessed as busy, the station needs to wait until the carrier becomes idle and then wait more for a period known as the Distributed Inter- Frame Space (DIFS). After the DIFS period is passed, the station again waits for a random back-off interval and then transmits if the medium is still free. After a directed transmission (unicast data frame) is correctly received, the receiving station sends an ACK frame back after a Short Inter-Frame Space (SIFS). The reception of an ACK frame following the transmission of a DATA frame notifies the transmitter that its data has been received by the receiver without error. If no ACK is received after the transmission of a data frame, the transmitter schedules the data frame for retransmission. In addition to the above basic transmission mechanism, the DCF employs an optional reservation based collision avoidance mechanism for unicast data packets. This option requires the sender and the receiver to exchange short Request-To-Send (RTS) and Clear-To-Send (CTS) control frames, respectively, prior to the actual data frame transmission to reserve the channel. Any stations which hear either the RTS or the CTS block their own transmissions (if any) to yield to the communication between this sender and its receiver. Each station maintains a timer called the Network Allocation Vector (NAV) which tracks the remaining time of any on-going transmissions of other stations. After a station receives a RTS, CTS, DATA, or ACK frame not destined for itself, it sets its NAV according to the duration field of the frame. The duration field contains the frame sender s estimation for how long the whole data frame delivery message exchange sequence (including all the SIFS waits and the acknowledgement) will take, or in other words, the reservation duration of this data frame delivery. After set, NAV may be extended if a newly received frame contains a duration field pointing to a later completion time. Figure 1 illustrates how nodes set their NAVs during a RTS-CTS-DATA-ACK handshake. Checking one s NAV before a station attempting to transmit is also known as virtual carrier sensing. If the NAV is not zero, the node needs to block its own transmissions to honor the channel reservations. In summary, a node blocks its own transmissions if either physical carrier sensing or virtual carrier sensing returns channel busy. 3

4 Nodes located between R and I NAV (EIFS) NAV (EIFS) NAV (EIFS) NAV (EIFS) Nodes located witihin R Source DIFS Contention Window RTS NAV (RTS) NAV (CTS) Data Destination SIFS CTS SIFS SIFS ACK 2.2 Capture Effect Figure 1: IEEE DCF Mechanism When a frequency modulation scheme, such as the Direct Sequence Spread Spectrum (DSSS) used by most IEEE and b physical layer (PHY) implementations, is used in wireless communication, an effect known as the capture effect [3, 17, 18, 15, 9] may occur. When two transmissions sent by two different transmitters at the same frequency overlap in time space, and they are received at the same receiver, the signals of the stronger transmission will capture the receiver modem, and signals of the weaker transmission will be rejected as noise. Different works (e.g., [9, 6, 16, 21, 20]) studied the analytical and simulation models for characterizing the capture effects. Among the results of these previous works, we adopt a simple yet widely accepted model to describe the capture effect. In our model, a receiver captures the signals of a particular transmission if the received energy P r of this transmission sufficiently exceeds all other received energy P i of n other concurrent interfering contenders combined by a minimum ratio. That is, the capture occurs when: n P r > α P i (1) i=1,i r This minimum ratio α is called the capture ratio. The received signals are assumed to have phase terms varying quickly enough to allow incoherent addition of the received power of each frame. WLAN systems such as the IEEE do not pay special attention to capture effects mainly to keep the system simple. Also the contention based MAC protocol largely reduces the time and space overlapping of concurrent transmissions. Nonetheless, the capture effect still exists in IEEE DSSS networks and it has been confirmed by several published studies. Authors in [11, 22, 21, 20] have also studied the impact of capture effect on traffic fairness and throughput of UDP and TCP flows for both ad hoc and infrastructure modes of WLAN systems. Another aspect, which has not been raised by many of the previous works, that 4

5 R I Figure 2: Example of network with 4 nodes, where R is transmission range and I is the carrier sense range needs some discussion before we proceed further is the capturing of a signal versus capturing of a frame. We consider the case when the new (stronger) frame arrives after the receiver begins to receive the weaker frame. A receiver being able to capture a stronger signal does not necessarily mean it can capture the stronger frame. Whether a receiver can capture a stronger frame also depends on several other factors such as: the arrival moment of the beginning of the stronger frame, the current receiving state of the receiver, the capability of the receiver to realize that it is seeing the beginning of a new (stronger) frame, and capability of the receiver to jump to the appropriate receiving state for beginning to process the new frame. If the receiver is not able to realize that it has just seen the beginning of a new frame and reset its receiving state accordingly, the bits of the new frame may be interpreted as the bits of the weaker frame, which typically result in failure of the frame s forward error checking and frame rejection. We are interested in capture effects because we believe that it can be used to our advantage to improve channel sharing efficiency. Consider the following example as shown in 2. Two concurrent connections share the same wireless communication channel. The first connection is from station 2 (source) to station 1 (destination) and the second is from station 3 to station 4. In current IEEE DCF, whichever connection acquires the channel first gets to complete its data frame delivery message exchange because stations of the other connection would have detected the carrier signals of this connection, or received reservation messages (RTS/CTS) of this connection, and remain blocked. However, if the stations are positioned in such a way that the energy levels of station 3 and 4 s transmissions as measured at station 1 and 2 are not strong enough that station 1 and 2 can still capture each other s transmissions, stations of the second connection should be permitted to communicate, even after stations of the first connection have begun their frame exchange. Similarly station 1 and 2 can do the same if station 3 and 4 have acquired the channel first. 5

6 A(x) m' v x R r s m x' dm I Figure 3: Capture analysis where x = x α and m = αm Because IEEE uses a combination of physical carrier sensing and channel reservation, and these mechanisms are generally effective in reducing packet collisions, the protocol is rather pessimistic and not very efficient in channel use because it does not encourage enough concurrent transmissions. Different researchers [10, 23, 13, 2] manipulated the functionality of RTS/CTS in order to enhance network performance either by reducing the collision rates or increase the channel spatial reuse. However, within the solutions proposed by [10, 2, 13], capture effect was not considered in the analysis, and [23, 2, 13] built complex mechanism over assumptions that do not reflect the complexities of WLAN system and wireless communication well. It is the above stated observations and inspirations from various related works which lead us to our own modification to the IEEE DCF protocol. We name the modification Location Enhanced DCF (LED). 3 Analysis of blocking probabilities with capture effect In this section, we perform an analysis on the probabilities of successful transmission in the presence of sensed signal(s). Using this probability, we illustrate the space for improvement to the original DCF in terms of over-pessimistic blocking of transmission. A free space omni-directional propagation channel model [19], in which many channels, especially outdoor channels, have been found to fit in practice is assumed. In 6

7 this propagation model, the received signal power, P r, is calculated as the following: P r = { P t Gt G r λ 2 (4 π) 2 D 2 L D D cross (2) P t G t G r h 2 t h2 r D 4 L D > D cross where P t is the transmission power, Gt is the transmitter antenna gain, Gr is the receiver antenna gain, D is the separation between transmitter and receiver, h t is the transmitter elevation, h r is the receiver elevation, L is the system loss factor not related to propagation ( 1), λ is the wavelength in meters, and D cross is calculated as D cross = (4 π h r h t )/λ. The first sub-model of Equation 2 is called the Friis free-space propagation model and only used when the distance between the transmitter and receiver is small. The second sub-model is called the two-ray ground reflection model and used when the distance is large. We assume that stations are uniformly distributed over an area with a density of δ. Each station has a transmission range R and a carrier sense range I, with the former being the range within which frames sent by the station can be received and decoded, and the latter being the range within which transmissions of the station can be detected (carrier busy). For the ease of analysis, we assume that all stations have the same traffic model. All data packets are of the same length. Each packet requires transmission time τ, and is randomly destined to a local neighborhood node. One data packet is generated at a randomly selected time within every time interval T, where T > τ. We also assume that all transmitters use the same transmission power and all antenna gains are the same. We are concerned about the scenarios where a node v may cause interference to another node r which is receiving a data packet delivery from node s as shown in Figure 3. Node v transmits only if its transmission doesn t affect the reception of DATA and ACK frames at nodes r and s respectively. Using the Friis radio propagation model as in equation 2 and receiver capture model as in equation 1, to allow nodes s and r to capture correctly each other s packets in the presence of any transmission from node v, the following should hold: (v.s > α s.r) AND (v.r > α s.r) (3) where a.b is the distance between node a and node b, and α is the capture ratio. we only use the Friis propagation model for the sake of analysis simplification. Figure 3 illustrates the situations for both r to capture s transmissions (DATA) and for s to capture r s transmissions (ACK) in the presence of v s transmission. For r to capture s transmissions, given m being the distance between s and r, the distance between v and r must be greater than αm. For s to capture r s transmissions, given x x being the distance between v and s, r must be within a circle of radius min(r, α ). Considering both conditions, r must be located within the shaded area A(x) in the figure. Hence, the probability that v s transmission doesn t corrupt communication between s and r is: P (B x) = A(x) πr 2 (4) 7

8 where the area A(x) is calculated as follow: A(x) = min(r, x α ) 0 2(π arccos( x x 2 m 2 +( αm) 2 2x m ))m dm (5) By unconditioning x, since we only worry about potential interferers within the carrier sensing range, we obtain: P (B) = I 0 A(x) 2x πr 2 dx (6) I2 Based on the traffic model, the probability that none of the nodes within the carrier sensing range of a node will transmit is obtained by: P 1 = [1 τ T ]δπi2 (7) and the probability that v s transmission will not interfere with other transmissions (if any) in the interference range is: P 2 = [1 τ T + τ T P (B)]δπI2 (8) Therefore, the probability that v can transmit with the presence of a nearby transmission without corrupting this transmission is given by: P b = P 2 P 1 (9) Note that the calculated P b is still conservative because of the following two assumptions: 1. Only the Friis propagation model is used in analysis because we assume I < D cross. However, in practice I may be greater than D cross and thus the distance x could also be greater than D cross. In this case the two-ray ground model should be used instead, which further reduces the probability of the interference and consequently increases the P b. 2. In the analysis, for simplicity, we assume that all nodes in the vicinity of v have the freedom of transmission. We do not take into accounts that some of these nodes will have to block because of other ongoing transmissions in their vicinities. Accounting for these blocked nodes would reduce P 1 and hence increase P b. We verified our analytical results by generating random network topologies and traffic patterns and studying the interference situation in each case. We also study how our simplified assumptions stated in the previous paragraph affect our blocking probability estimation by relaxing them in simulation runs. 8

9 1 0.8 Anl. P_2 Anl. P_1 Anl. P_b Sim. P_2 Sim. P_1 Sim. P_b Probability No. of Nodes Probability Figure 4: Blocking Probability tau/t=0.01 (with assump.) tau/t=0.01 tau/t=0.02 tau/t=0.03 tau/t=0.04 tau/t=0.01 tau/t= No. of Nodes Figure 5: P b with different τ/t values 9

10 For constructing each random network, we place the v node at the center of an area of Transmitter nodes are distributed uniformly in this area. Then a position is picked within the simulation area and uniformly within the circle with radius R centered at each transmitter for its corresponding receiver to establish a connection. Then each transmitter starts transmitting following the traffic model described before: all packets require transmission time τ and they are generated randomly at a constant rate: one packet every time interval T, where T > τ. When v has a frame to send, we study if it will be blocked under the current IEEE operations and when blocked if indeed v s transmission will collide with other communications. The number of situations where unnecessary blocking is suggested by IEEE is then divided over the total number of simulated situations to derive the probability of unnecessary blocking, which is compared to the analytical result. Figure 4 plots both the analytical and the simulated values of P 1, P 2, and P b for R=250, I=550, α=5, τ/t =0.01, and different numbers of nodes (thus varying the node density δ). As we can see, the simulation results closely match the analytical results which validates our analysis. Figure 5 plots the simulation of P b with the simplification assumptions relaxed. Also this figure plots P b with different packet load values. The P b plot for with these assumptions, which is copied from Figure 4, is also include for easy comparison. The plots show that our analytical results are conservative. As expected the probability analyzed above only takes into account whether the channel assessing node s transmission may corrupt other ongoing data deliveries. It does not address if this channel assessing nodes transmission will be received correctly by its receiver. Such a transmission may still fail at its receiver if other ongoing data deliveries produce enough interfering energy there. The above analysis shows that the unnecessary blocking probability of DCF is large enough (as high as 35%) to motivate us to consider modifying the MAC layer to exploit the capture phenomena of the physical layer. In the following section we will describe the newly proposed IEEE DCF modifications. 4 Location Enhanced DCF for IEEE In this section, we describe our Location Enhanced DCF (LED) for IEEE Before we introduce our approach of using location information and capture effect to improve channel efficiency, several terms which will be used during the description need to be clarified to avoid confusion. In our description, we use the term delivery for the whole handshake procedure for delivering a unicast packet. Depends on the packet size, a delivery may involve the full RTS-CTS-DATA-ACK 4-way message exchange sequence or just DATA-ACK 2-way exchange. A source is the station having data to send during a delivery. The destination of a delivery is the station to whom the source wishes to send data. The sender and receiver are the sender and receiver of individual RTS, CTS, DATA, or ACK frame. Transmitter is used interchangeably with sender. Connection is used to refer to both the source and destination stations collectively. 10

11 4.1 Protocol Overview Our approach is simple: to include more information about each transmission in the transmission so that any other stations hearing the transmission are able to better assess whether their own transmissions may collide with this transmissions. Among various transmission related parameters, the locations of the transmitters and receivers are the most important. We assume that each node is capable of acquiring its own location, e.g. GPS or other RF based localization methods. A station can retrieve other communication parameters regarding its own transmitter/receiver easily as they are typically configuration parameters. At a given station, for a particular on-going delivery that does not involve itself, if the locations and other communication parameters such as transmission power and antenna gains of the source and destination stations are known, using the propagation model 1 in equation 2 this station can compute the received energy level of the frames of the data delivery at their receivers. Then if the capture ratio of the receiver is also known, knowing its own location, antenna gain, and transmission power, this node can make a prediction of whether its own transmission may interfere this on-going data delivery. This is a rather simplified estimation model as each channel assessing station only considers the effects from its own potential transmission. It may occur that several stations simultaneously predict that their own transmissions will not cause collision to the on-going delivery. In this event, the aggregated energy from all these side transmissions may actually change the result of the capture effect and cause enough interference with the ongoing delivery. We postpone this issue to future studies as we currently expect that the probability of this happening is low. Another issue is that in the current model, a station only is concerned about if its own transmission will affect an on-going delivery. It does not consider if its own transmission can be received correctly by its destination. This optimistic approach is largely for keeping the model simple at its current stage. Studies for the impact of this additional estimation refinement are also deferred. 4.2 PHY As we point out earlier, the current standards do not require a WLAN receiver to be able to capture a new (stronger) frame after the receiver has been tuned to receive another frame, even if the signals of the new frame are strong enough to be captured. As we explained before, unless the capture capability is specifically designed into WLAN receivers, they are usually not able to correctly capture the new frame. This may cause problems in our approach. If a station decides to transmit after it estimates that its own transmission will not interfere with an ongoing delivery, it will begin to send its own frame. However, chances are that the intended receiver of this frame is already engaged in receiving another frame, one of the frames of the on-going delivery. As a result, this receiver will not receive and interpret the new frame correctly even if the signals are strong enough. 1 This model can be easily changed to any other propagation model depending on the operation environment of the system without affecting how the protocol operates. 11

12 Figure 6: Packet Capture of Below is a more detailed analysis of what may happen at a receiver which do not provide support for capturing of a stronger frame if it arrives after the receiver has begun receiving a weaker frame. Depending on at what exact moment the beginning of the stronger frame arrives during the reception of a weaker frame, there are three different situations as shown in Figure 6. Each IEEE DSSS frame has three sections based on their affects on receiver s physical and MAC layer operations: the Physical Layer Convergence Protocol (PLCP) Preamble section is used to train the receiver modem for synchronization; the PLCP Header section contains medium (DSSS) dependent information such as modulation choice and frame length; and the PSDU section contains the actual MAC layer data. 2 Accordingly, the reception of a frame is also broken into three stages. 1. If the stronger frame arrives during the training period of the modem towards receiving the weaker frame (stage 1), the modem is able to get retrained and switch to reception of the new stronger frame. 2. If the stronger frame arrives during the reception stage of the weaker frames PLCP header, or stage 2 in the illustration, the new signal would likely destroy 2 In the original IEEE standard, this section is called MAC Protocol Data Unit (MPDU). The later IEEE b standard changes it to PSDU (PLCP Service Data Unit). 12

13 First Transmission Second Transmission Recepition t Receiverenergyincrease Figure 7: Message In Message the data contained in the weaker frames PLCP header and result in PLCP reception error or CRC failure, in which case the receiver goes back to idle state. Then, if this happens soon enough, the receiver may still be able to detect the new carrier for the stronger frame and get trained for receiving it, if the stronger transmission is still in its SYNC portion of the frame. 3. If the stronger frame arrives during the reception stage of the weaker frames PSDU (stage 3), it would most likely destroy the reception due to two major reasons. First the demodulation algorithm the receiver is currently engaged in for the weaker frame may be different from the modulation used by the stronger frame. Second the bits of the stronger frame, even correctly demodulated, are interpreted as part of the PSDU of the weaker frame and passed up for MAC processing. After the whole message is received, the MAC forward error detection mechanism will fail and the frame is dropped. Unless the stronger transmission is still in its SYNC section and the receiver can catch it quick enough, the stronger frame will not be received correctly either. For nodes that have received a frame in error, obviously they can not determine the duration of the reservation, they need to wait for an Extended Inter-Frame Space (EIFS) after the carrier becomes idle. The EIFS will leave enough time for the on-going frame exchange to finish. Fortunately, WLAN receiver designs which do support the capture of a new frame after the receiver has already begun to receive another frame do exist. One example of such a receiver PHY design is Lucent s PHY design with Message-In-A-Message (MIM) support [5]. In this design, the newly arrived frame is referred to as the (new) message in the (current) message. A MIM receiver is very similar to normal WLAN PHY designs. Except that it continues to monitor the received signal strength after the PHY transits to data reception state. If the received signal strength increases significantly during the reception of a frame, as shown in Figure 7, the receiver considers that it may have detected the beginning of a MIM frame and hence switches to a special MIM state to handle the new frame. While under the MIM state, the receiver tries to detect a carrier for a new frame. It the carrier signal is detected, the receiver begins to receive the initial portion (namely 13

14 LOCT PWRT GAINT LOCR PWRR GAINR DUR Header Body FCS SYNC SFD Signal,Service,Length ENH CRC MAC Frame Preamble PLCP Header MPDU/PSDU Figure 8: Frame Structure the preamble) of the new frame and gets retrained to be synchronized with the new transmission. If no carrier is detected, which means the energy increase is caused by noise, the PHY will remain in this MIM state until either a carrier is detected or the scheduled reception termination time for the first frame is reached. With a MIM capable design, a WLAN receiver is always able to correctly detect and capture a strong frame regardless the current state of the receiver, unlike in regular PHY designs the strong frame can only be correctly detected and captured while the PHY is under certain states during its reception of a weak frame. 4.3 MAC Our enhanced design for a DCF WLAN MAC is atop of a MIM capable PHY. Figure 8 shows the frame format to support the enhanced functionalities of the new MAC. We propose to insert a block of information called ENH ( Enhanced ) to provide the additional information needed for LED. Since the earlier the ENH block is received, the sooner the receiver can decide if it needs to block its own transmission, the ENH block should be inserted before the true MAC data section, or the PLCP (Physical Layer Convergence Protocol) Service Data Unit (PSDU). However, inserting ENH before the end of the DSSS PLCP header or at the beginning of the PSDU needs a little more thinking. In current design, we have the ENH as part of the PLCP header mainly due to the fact that firstly the PLCP header has its own CRC field so the contents of the ENH block can immediately be verified and utilized, and secondly all stations within the service set can understand the ENH block since the PLCP header is transmitted at the base rate. The ENH block is further divided into seven fields. The LOCT field contains the location of the frame transmitter, the PWRT field describes the transmission power of the transmitter, and the GAINT field specifies the transmission antenna gain. The LOCR, PWRR, and GAINR fields contain the same pieces of information for the receiver. The DUR field is a copy of the Duration field of a RTS, CTS, DATA, or ACK message MAC format, except that it is in PLCP header instead of PSDU. When a source has a unicast data packet to send, it starts by sending out an RTS message to reserve the channel. In this message, the source fills the LOCT, PWRT, and GAINT fields with its own parameters, and the LOCR, PWRR, and GAINR with the destination s parameters. If these parameters are not known at that time, they are set to NIL. Upon receiving the RTS, the destination of the data packet copies the LOCT, PWRT, and GAINT fields into the corresponding fields of its CTS message. It also fills 14

15 LED PHY_DATA. ind(data) MAC PHY_CCA. ind(busy) PHY_RXSTART. ind(rxvector) PHY_DATA. ind(data) PHY_RXEND. ind(rxerror) PHY_CCA. ind(idle) PHY PLCP PMD_ED/ PMD_CS PMD_DATAi nd PMD_DATA. ind PMD_DATAi nd PMD_DATAi nd not PMD_ED/ PMD_CS PDM_RATE.req PDM_RATE.req PMD_MODULATION.req DSSS PHY PMD SYNC SFD Signal,Service,Length ENH CRC Data Preamble PLCP Header MPDU/PSDU Figure 9: PHY-MAC Interactions the LOCR, PWRR, and GAINR fields of the CTS message with its own parameters. In subsequent DATA and ACK messages, full descriptions of both the source and the destination are included. In case of the frame size being less than the RTS/CTS threshold and no RTS/CTS handshake being conducted, the DATA message will have its fields set in the same fashion as the RTS message, and the ACK message is filled the same way as the CTS message. In the standard IEEE , the PHY (PLCP in particular) does not deliver any data bits to the MAC layer until the PSDU reception has begun. Then the receiver will proceed all the way till the end of the message (unless the carrier is lost in the middle of the reception). Received bits are passed to the MAC layer as they are decoded for being assembled into the MAC frame. At the end of the PSDU is a forward error detection CRC block called FCS. If the MAC frame passes the CRC check, it is accepted and passed up for further MAC processing. If the CRC fails, the frame is dropped. In LED, because of the ENH block s location, additional data namely the ENH block needs to be passed from the PLCP layer to the LED part of the MAC layer for processing. The signal and data interactions among the layers are illustrated in Figure 9. A parameter cache may be maintained by stations to store the location, power, and antenna information of already known stations. This way when sending data to a station in cache, the cached parameters may be used in the corresponding fields of the ENH block instead of NIL values. Cache entries are updated if newer information is received from their corresponding stations. Cache entries are removed after its expiration time. Upon receiving a frame with ENH block, a station other than the intended receiver of the frame begins interference estimation. That is, if this station s own transmission will cause interference to the delivery which the just received frame belongs to. The station needs to calculate the power level of its own transmission at both the source 15

16 Pi s and destination P i d of the ongoing data delivery using Equation 2. The station also needs to calculate the received power level of the destination at the source Pd s and that of the source measured at the destination Ps d. If (Pd s > αp i s) and (P s d > αpi d ), the station does not block its own transmissions. Otherwise, it blocks its transmissions. In the case that the communication parameters of either the source or the destination are unknown, the assessing node assumes the worst and blocks its own transmission. Normally, after a station other than the intended receiver of the frame receives a RTS, CTS, DATA, or ACK message, it needs to set its NAV value according to the Duration field of the message, which is set to the time required for the full RTS-CTS- DATA-ACK message exchange to finish. In LED, a non-receiver station will only set the NAV according to the standard when it determines that its own transmission will interfere with this on-going delivery. Otherwise, the NAV is not set. In addition, if the non-receiver station determines that it does not need to block it own transmission to yield to the on-going delivery, it also needs to set a special vector called CCA-Suppression Vector (CSV). The CSV is a suppression timer. It is set according to the Duration field of the received RTS, CTS, DATA, or ACK message, which means the timer will run till the whole on-going delivery is completed. Together with an active CSV each station needs to remember the source and destination stations of the delivery the CSV is referring to so that this station will block its own transmissions addressed for either. Just like NAV, if a later message prolongs the duration of the delivery, the CSV can also be extended. Once the CSV counts down to zero, a CCAReset signal is issued to the PHY layer so the CCA mechanism will retest the carrier and report the result. Upon receiving a busy CCA indicator, the station will block until either the carrier is idle again, or another frame is captured. On the source or destination station of the on-going delivery, according to the standard, the NAV is not set for the duration of the delivery. In LED, this specification is still followed. However, a LED receiver does set its CSV to the end of the delivery. The reason is that since LED permits concurrent transmissions by other stations as long as they do not produce enough energy to disturb the on-going delivery. Thus, if any other station is indeed transmitting, their carrier may cause the source and destination of the on-going delivery to abort their RTS-CTS-DATA-ACK handshake. Thus, the CCA should be suppressed on the source and destination stations till the end of their delivery. In total, a LED station has three indicators related to the channel estimation. The CCA is the physical carrier indicator. It is TRUE when the PHY layer detects carrier (or energy exceeding threshold, or both, depending on equipment vendor implementation). The NAV indicator is the virtual carrier indicator. It is TRUE when there is a reservation which needs to be honored. That is, if this station transmits, then the transmission will interfere with the on-going delivery. Finally, the CSV indicator tells the station if it should ignore the CCA. It is TRUE when the suppression timer is running. The decision of whether this station should block its own transmission is made as follows. if ((CCA AND (NOT CSV)) OR NAV) then BLOCK (10) Since the CSV is implemented in MAC layer, even when Equation 10 indicates not to block, the PHY layer may not have the PHY TXSTART signal enabled if it has engaged in receiving. Thus, a PHY reset signal is needed to force the PHY to leave 16

17 the receiving state and enable PHY TXSTART signal when the MAC has a frame to send. Following the reset, the PHY layer enters transmission state to send out the frame. However, on the source or destination station of an ongoing delivery, if during the SIFS gaps within the message exchange sequence a station detects a carrier and begin to receive a new frame, it needs to block till its PHY to finish the PLCP header part of the packet. Then the test of Equation 10 is carried to see if continuing with the data delivery message sequence will cause frame collision at the receiver of this new frame. Only if the test result is no blocking, the delivery sequence proceeds. Another issue occurs if a channel assessing node only detects carrier but can not decode the frame. In this case this node is not able to estimate whether its transmission will affect this on-going transmission. Either an aggressive approach or a conservative approach can be taken. In the aggressive approach this node will not block its own transmission in the event of detecting a carrier but not being able to decode the frame, while in the conservative approach this node will block its own transmission. A tricky issue requiring more discussion is for a station to receive messages from more than one delivery. We refer to this situation as the stacked delivery. Simple stacking situations can be handled by using CSV and NAV together. If the first delivery does not require this station to block its own transmission (non-blocking delivery), the NAV is not set and the CSV is set till the end of this delivery. Now the second delivery is started and it is a blocking delivery. In this case, the NAV is set immediately to the end of the second delivery, and the CSV is not changed. The overall result is the station will block from the moment the first frame of the second delivery is received till the completion of the second delivery. The opposite case is that the first delivery is a blocking delivery and the second is non-blocking. Now the NAV is set for the first delivery and the CSV is set for the second. Overall, the station only blocks till the end of the first delivery when the NAV is cleared. More complex stacking situations are left for future works. 5 Performance Evaluation In this section, we present extensive simulation-based studies on the performance of the LED mechanism. The performance comparisons were done using the ns-2 simulator, enhanced with the CMU-wireless extensions (the underlying link layer is IEEE with 11 Mbps data rate). In doing this, we extended ns-2 as follows: We modified the capture model to allow receivers to capture the stronger packet out of the weaker packet(s), as in Equation 1, if the stronger packet comes after the weaker to reflect the PHY design as discussed in the previous section. Current implementation of ns-2 allows the node to compare the newly coming packet only with the one it is receiving. In order to implement the capture Equation 1, we extended the PHY layer in ns-2 to allow each node to keep track of all its incoming packets and the aggregated background signals. Also in order to create a more realistic environment, we allow each node to aggregate the signals that have lower values than the CSThresh used by ns-2. 17

18 We enhanced the IEEE MAC layer by extending it with the implementation of our LED mechanism. Each of our simulated networks consists of a set of connections which are constructed as pairs of stationary sender and receiver nodes. The senders and receivers are placed in a 1000m 1000m area in the same fashion as the simulations described before in Section 3. We assume that each sender has already cached the location of its corresponding receiver. Other parameters such as transmission power levels and antenna gains are also assumed to be fixed and known to all stations therefore not included in simulation. In simulation, the ENH header only contains LOCT and LOCR fields of 32 bits each. The transmission radius R of a node is selected to be 250m while the interference radius is 550m. Each connection is a UDP flow with packets of 1000 bytes which are transmitted at 11Mbps. To simplify the simulation implementation, all RTS and CTS messages, as well as the PLCP headers, are also sent as 11Mbps. Such a simplification should not affect the correctness of the evaluation method since we are more interested in relative performance improvement. Each simulation is run for a fixed duration of 50 seconds. Each point on the curves to be presented is an average of 5 simulation runs. IEEE equipment does not come with capture ratio specifications. The capture ratio used in simulation is derived by the following method. Given a specific Bit Error Rate (BER) the theoretical required Signal to Noise Ratio (SNR) for a particular modulation technique can be calculated. In the case of 11mbps CCK modulation, according to calculations described by [19], it can be determined that 18dB of SNR is needed to achieve 10 8 BER, as specified by Orinoco s WLAN cards. The 11 mbps CCK uses 8 chip/symbol, which is 9dB spreading gain. In addition, CCK coding provides about 2dB additional coding gain. All together the processing gain is 11dB. When only considering signals before receiver processing, the SNR requirement is 7dB. Roughly, this maps to 5 times signal power over interference. We adopt the same number as the capture ratio. In our model, when a station is receiving frame A and frame B arrives, if the received power of frame A, P A, is more than 5 times P B, the receiver captures A and continuously receives frame A; if P B is more than 5 times of P A, the receiver captures B and drops A; and in all other situations, packets collide and no frame is received. We modeled various scenarios of different node densities, work loads, transmission and interference ranges (transmission power levels), and errors in location estimation and their effects on performance. To study the performance of our suggested schemes, we compare our LED with both the Original IEEE and MACAW protocols 3. As described in Section 4, we experiment with two different flavors of LED: LED CS and LED RX. LED CS mechanism is an aggressive (optimistic) version of LED mechanism in which a node receiving a frame with signal level lower than RXThresh, from an on going transmission, assumes its transmission will not interfere with that ongoing transmission and therefore should not block. On the other hand, LED RX is a conservative (pessimistic) version of LED in which a node assumes its transmission will interfere with the ongoing transmission of a frame with a signal lower than RXThresh. For evaluation, we take the following measurements: 3 Both Original and MACAW protocols use the extended ns-2 capture model as described earlier. 18

19 Node 4 ENH header Blocking period SIFS CTS Node 3 Node 2 RTS RTS SIFS RTS SIFS Data Node 1 SIFS CTS Timeout (SIFS+Data) Figure 10: Medium under utilization scenario of LED mechanism 1. Effective Throughput: This counts the total number of packets received by all the receiver nodes over the simulation period. 2. Collision Packets: This counts the total number of collisions that involve data packets by all the node over the simulation period. Note that this metric considers the collision of the data packets at all nodes, not only destination nodes. 3. Fairness Index: To measure the bandwidth sharing of the connections under different mechanisms, we use Jain s fairness index [7, 12] which is defined as the following: F = ( N i=1 γ i) 2 N (11) N i=1 γ2 i where N is the number of connections and γ i is the number of received packets for connection i. We start with experimenting using RTS/CTS access mode. Although the LED mechanism forces each node to be blocked during the ENH header of each received frame, we found that forcing the node to be blocked during the RTS/CTS period of the other connections will increase the network throughput. The reason for this is more related to the particular ns2 implementation of the physical layer. To explain this, consider Figure 2 in which each node of node 2 and node 3 has packet to send to node 1 and node 4 respectively. Assume node 2 would start the transmission cycle by transmitting RTS packet to node 1 ashown in Figure 10. When node 1 receives the RTS packet, it waits for SIFS period and, if the medium is still idle, transmit the CTS back to node 2. After transmitting the CS packet, the ns2 implementation of node 2 sets a timer for period equal to the SIFS period plus the period for transmitting the data packet. In ns2 implementations, node 2 is expecting to receive the data packet within this period and if it timeouts with no received data packet, it indicates the failure of the current transmission cycle. Meanwhile, as shown in Figure 10, node 3 detects the CTS packet and figure out that its transmission will not affect the on going transmission and hence decided to start its own transmission cycle by transmitting the RTS packet that happened to be within the SIFS period after the CTS packet of node 1. Since LED mechanism 19

20 Original MACAW MACAW Effective Throughput Enhancement Percentage # of Connections # of Connections Figure 11: Effective throughput versus node density using RTS/CTS access mode. Figure 12: Throughput enhancement over Original protocol versus node density using RTS/CTS access mode. forces each node to be blocked during the ENH header of each received frame, node 2 will be blocked during the ENH period of the RTS of node 3 which happen to last more than the SIFS period. Therefore, node 2 detects the failure of its current transmission cycle and tries to start another transmission cycle by sending new RTS packet to node 1 as shown in the Figure after waiting for DIFS and a doubled contention window. However, during the ns2 implementation, node 1 which is expecting data packet will drop such RTS packet. Node 2 will timeouts and detects an unsuccessful transmission and then tries again to send RTS packet after the DIFS and new doubled contention window. Node 1 will drop any non data packet (e.g. RTS packets) while it is waiting for its timeout to expire. This mechanism/implementation under utilizing the medium and hence reduce the network throughput. IEEE standards do not specify how a node should react when it receives non data packet while it is expecting to receive a data packet during a certain time period. To enhance the network performance by eliminating such problem, we forced each node to be blocked during the transmission period of RTS/CTS cycle. This could be done by including the blocking duration information in the ENH header or set one of the locations in ENH to null to force the nodes to be blocked for the whole packets and then set NAV to the end of RTS/CTS cycle instead of the while transmission cycle. Back to Figure 10, with such mechanism, node 3 will be blocked from transmission until the transmitting the ENH of the data packet by node 2 and therefore the on going transmission cycle will not be disturbed. Figure 11 shows the effective throughput of the networks with different numbers of connections. The UDP flows are constant bit rate (CBR) at a rate of 20 packets per second. LED CS, LED RX, MACAW have higher throughput than the original IEEE DCF. Figure 12 further illustrates the improvements by showing the percentage throughput gain of LED CS, LED RX, and MACAW over Original. At their peaks, LED CS could reach about 20% more than the Original amd LED RX could reach to 22% higher throughput than Original while MACAW could reach to 8%. LED RX experiences higher throughput than LED CS for scenarios especially with the number of connections is large because of the aggressive nature of the LED CS mechanism, which also leads to high number of collisions and retransmissions. The aggressiveness 20